In a light information storage apparatus such as an optical disk apparatus, a laser light is condensed and radiated onto an information storage medium such as an optical disk. The light that has been obtained by that laser light's being reflected from the information storage medium, or the light that has been obtained by that laser light's being diffracted by the information storage medium, or the like, is received by a photodetector. This photodetector is divided into at least two detection parts, and, by each of the respective detection parts, the light is converted into a corresponding electric current signal. By determining the sum of, or the difference between, those electric current signals, there is obtained a track error signal (Track Error Signal: TES) or a focus error signal (Focus Error Signal: FES) or the like. By using those signals such as a track error signal, focus and tracking are controlled.
In, for example, a method of obtaining a track error signal called “the Push-Pull Method”, there are utilized ±primary diffracted light rays that are obtained when a laser light has been radiated onto the track guide groove on a relevant optical disk. The relative intensities of respective ones of the ±primary diffracted light rays vary depending on the state of the laser light spot's being applied to the track guide groove. Therefore, by the ±primary diffracted light rays' being respectively received by the two-divided photodetector and converted into respective electric current signals and by being determined the difference between those electric current signals, a track error signal is obtained.
Meanwhile, in the light information storage apparatus accessing a rewritable type optical disk, a more intense light is radiated onto the optical disk when performing write-in or erasure than when performing read-out. For this reason, the level of the electric current signal obtained from the photodetector greatly fluctuates when the write-in or erasure operation is switched to the read-out operation or vice versa.
Also, in a case, as well, where a change in the amount of light occurs due to the variation in the reflectance of the optical disk, the signal level of the electric current signal obtained from the photodetector fluctuates.
In the light information storage apparatus accessing an optical disk that is rotated with the angular velocity thereof being fixed, since the linear velocity is different between the inner periphery and the outer periphery, the intensity of the laser light is varied in harmony with the change in the linear velocity. Also, in a general type light information storage apparatus, in order to make the writing-in ability stable, the intensity of the laser light that is used when performing write-in is adjusted to a value corresponding to the environmental conditions such as temperature.
Accordingly, when the difference signal between the two photodetector outputs is simply used as a track error signal, the control becomes unstable when the level of the electric current signal changes due to the change in the intensity of the laser light or the change in the amount of it.
In view of the above, conventionally, a normalization circuit is incorporated into the optical disk apparatus, and, by the normalization circuit, the track error signal is normalized. Into this normalization circuit, two input signals are input, and the difference signal between those two input signals is divided by the sum signal between those two, thereby normalization is performed. As a result of this, there is obtained a track error signal wherein the levels of the input signals have on principle been canceled.
Normalization like that is performed also when determining a focus error signal or the like.
By the way, in the normalization circuit, it is known that a predetermined error which is specific for each relevant circuit occurs not only when the circuit receives the input signals but also when the circuit determines the difference signal and sum signal. The predetermined error like that has effects upon the result of the normalization calculation and produces the normalization error. Normalization error like that has hitherto been ignored.
However, in optical disks which have been widely used in recent years, the track pitch has been made more dense in order to increase the density at which recording is done. So it is possible that even small track deviation will cause the occurrence of cross talk between the current track and a track adjacent thereto. For that reason, in order to accurately control focusing or tracking, it has strongly been demanded to accurately correct the normalization error of the normalization circuit. However, a correction method that enables accurate correction of the normalization error is not known.